Via structure and method thereof

ABSTRACT

A layered micro-electronic and/or micro-mechanic structure comprises at least three alternating electrically conductive layers with insulating layers between the conductive layers. There is also provided a via in a first outer layer, said via comprising an insulated conductive connection made of wafer native material through the layer, an electrically conductive plug extending through the other layers and into said via in the first outer layer in order to provide conductivity through the layers, and an insulating enclosure surrounding said conductive plug in at least one selected layer of said other layers for insulating said plug from the material in said selected layer. It also relates to micro-electronic and/or micro-mechanic device comprising a movable member provided above a cavity such that it is movable in at least one direction. The device has a layered structure according to the invention. Methods of making such a layered MEMS structure is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and is a continuation of U.S.application Ser. No. 13/141,609, filed on Jun. 22, 2011, now pending,and claiming priority to International Application NumberPCT/SE2009/051496 filed on Dec. 23, 2009, which claims priority toSwedish Application No. 0802663-5 filed on Dec. 23, 2008, the entirecontents of which are herein incorporated by reference as part of thisapplication.

FIELD OF INVENTION

The inventions disclosed herein relate generally to processes for themanufacture micro-mechanic devices comprising movable elements that canbe actuated electrically and/or electrostatically by means ofelectrodes. Examples of such devices are micro mirrors and large arraysof micro mirrors, micro-switches, oscillators, loud-speaker membranes.In particular the inventions relate to aspects of the processes thatenable closer spacing of mirrors and that provides structures thateliminate disturbances in the actuation of the mirrors in operation.Other applications are energy generating system, e.g. so called energyharvesting using piezo materials for transforming kinetic energy toelectric energy.

DISCUSSION OF BACKGROUND INFORMATION

The prior art devices and processes comprise lateral routing (surfacerouting) of electrical leads to and from the actuating structures(electrodes). It is difficult to space such surface routed electricalleads closely enough. Actuation of deflectable elements such as mirrorarrays at several positions requires often that the electrical leads arerouted beneath adjacent mirrors, which inevitably may cause disturbanceand interferes with mirrors in the arrays that it is not desired toactuate.

Furthermore, wire-bonding is often required on the same side of thewafer where the deflectable elements such as mirrors and arrays ofmirrors are provided, which also requires space, and prevents furtherreduction of dimensions and/or prevents closer spacing of mirrors in thearrays.

US 2004/0009624 A1 (Gormley et al) discloses a micro-mechanic devicehaving mirror arrays that are actuated by electrodes coupled through asubstrate by wafer through connections (vias). However, routings ofnon-wafer native material are made on the surface of the substrate, onthe opposite side of where the mirrors are provided.

In SE-526366 (Silex Microsystems) there is disclosed use ofwafer-through vias for actuating mirrors.

SUMMARY OF THE INVENTION

In accordance with the present invention vias are provided through thewafers so as to enable the provision of electrodes at desired locationsfor actuation purposes. Thus, it enables positioning of actuationelectrodes underneath mirrors without causing disturbance to othermirrors in the array. Routing of leads with deposited materials is thenmade possible on the backside of the wafer. Routing of leads is providedwithin the wafer native material in the wafer structure, by providinginsulated regions within the layers.

Thus, there is provided a layered micro-electronic and/or micro-mechanicstructure, comprising at least three alternating electrically conductivelayers with insulating layers between the conductive layers, and furthercomprising

a via in a first outer layer, said via comprising an insulatedconductive connection made of wafer native material through the layer;an electrically conductive plug extending through the other layers andinto said via in the first outer layer in order to provide conductivitythrough the layers; and an insulating enclosure surrounding saidconductive plug in at least one selected layer of said other layers forinsulating said plug from the material in said selected layer.

Furthermore, according to the invention, these actuation and routingprinciples are used for making micro-mechanic devices having movableelements that can be caused to deflect from a nominal or rest positionto position in which part of or the entire element is spatiallydisplaced. Such elements can be embodied as e.g. mirrors attached at oneend to the substrate by means of a “hinge structure” to render themdeflectable, or mirrors attached to the substrate in several, preferablyfour points, so as to be translatable rather than deflected. Thisentails moving the entire mirror plane in a parallel movement. Otherelements are loudspeaker membranes, which are rigidly attached along theperiphery to the substrate, and can be caused to vibrate. Also,oscillators for clocking purposes, e.g. crystals, are examples ofembodiments of the invention.

Also provided is a micro-electronic and/or micro-mechanic devicecomprising a substrate having a cavity formed therein and at least onemovable member provided above the cavity such that it is movable in atleast one direction, the device further comprising at least oneelectrostatic actuation electrode for each movable member for causingmovement of said member(s), wherein the electrodes are coupled to viastructures extending through the substrate, whereby the coupling betweenelectrodes and via structures is provided by means of a layeredmicro-electronic and/or micro-mechanic structure according to theinvention.

Also, if a piezo material is applied on the movable element it will bepossible to pick up energy from external mechanical motion. Thereby, twolaterally routed electrode layers insulated from each other andconnected to wafer through connections are provided on each side of theelement made of the piezo material.

Preferably, an insulating enclosure has a geometry such that theenclosure extends laterally within the layer, thereby forming a routingstructure for routing signals laterally within the layer.

Suitably, said plug is uninsulated from surrounding material in at leastone of the other layers.

In one embodiment there are three conductive layers and the insulatingenclosure is provided in one of the layers.

In other embodiments there are four conductive layers and the insulatingenclosure is provided in two of the layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be describe in more detail with reference to thedrawings which disclose non-limiting examples, and in which

FIG. 1 a shows schematically a device having a deflectable mirroraccording to prior art;

FIG. 1 b is another example of a prior art mirror device;

FIG. 1 c is a still further example of a prior art device;

FIG. 2 a-d illustrate a sequence for venting during manufacture;

FIG. 3 a-e illustrate embodiments of routing in layers;

FIG. 4 a shows one embodiment of a gimbal hinge with comb electrodeactuation;

FIG. 4 b shows one embodiment of a gimbal hinge with a combined combelectrode and plate electrode actuation;

FIG. 5 shows an embodiment with actuation using of hidden hinges;

FIG. 6 shows another embodiment with actuation using of hidden hinges;

FIG. 7 shows a gimbal hinge structure and actuation means

FIG. 8 is an embodiment for optical switching; and

FIG. 9 shows an embodiment with a movable element made form a piezomaterial.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Herein the term “via” is used for any structure that extends through awafer and is capable of transmitting electrical signals (the y can alsobe referred to as “wafer through connections”). These “vias” are made ofwafer native material, i.e. from the wafer it self of which they form apart. The vias can have different cross sections, i.e circular,rectangular, square or irregular, although in most cases circular crosssections are preferred.

The term “via structure” is taken to mean both a single piece ofmaterial extending into and/or through a wafer, and a combination ofdetails forming a more complex wafer through connection or a connectionextending at least partly through a wafer and/or through only one or afew of several layers in a wafer structure.

A “wafer structure” is taken to mean several wafers bonded together, oran SOI wafer having a handle layer, a buried oxide layer and a devicelayer, and any combination of wafers of various types forming at leastone layer separated from another, e.g. by an oxide layer.

For the purpose of this application “energized electrode” shall be takento mean an electrode to which a voltage has been applied versus ground(GND) for actuation of a movable element located near the electrode. Insuch an energized state the electrode becomes charged, but practicallyno current flows between the electrode and the element to be actuated,which will only be moved in the electric field generated.

Routing by Vias

A number of embodiments of the invention will now be described byreference to the drawing figures.

However, first some prior art structures will be discussed.

Thus, in FIG. 1 a there is schematically shown (not to scale) a priorart device, namely one mirror structure of a mirror array, havingrouting members.

The mirrors 1 and 2 are attached to a supporting post 3 via hingestructures 4 and 5, respectively, made by MEMS techniques in a substratewafer SW. Beneath each mirror there is provided actuation electrodes 6,7 that will cause the mirrors 1, 2 to deflect when the electrodes 6, 7are energized, i.e. charging the electrodes by applying a high voltage.The hinges 4,5 are torsion bars, i.e. in the shown device the mirrorswill tilt around an axis extending in the plane of the paper along thehinges. Thus, there is provided two electrodes on each side of the post3, i.e. one electrode in each pair is hidden by the one shown.

In this prior art device the electrodes are routed “away” from the arrayby electrical leads 8, 9 provided on the substrate surface. As can beunderstood, the lead 8 from electrode 6 for the actuation of mirror 1will have to pass beneath mirror 2, and when energized it will affectalso mirror 2 to some extent causing functional artefacts.

Another prior art device, shown in FIG. 1 b, employs vias 10, 11 throughthe wafer SW for energizing the electrodes connecting the electrodes 6,7 to routing leads 12, 13 on the back side of the wafer SW. The vias 10,11 are made of non-wafer native material, i.e. a hole has been made inthe substrate and then filled with metal or some other conductivematerial, such as aluminium or highly dope semiconductor. Routing leads12, 13 are of course provided adjacent each other, suitably in parallelto the periphery of the wafer where wire bonding can be provided, ifdesired. Alternatively there could be provided a double layer of metalon the back side with an insulating layer between the conductive layers.In this way one could provide crossing conductors thereby increasing theflexibility in the routing structures. By plating (or any other suitablemethod known by the skilled man) contact bumps can be provided.Preferably so called Under Bump Metallization (UBM), which enablesflip-chip mounting of the mirror component. Control circuits, e.g.ASICs, can thereby be mounted directly on the back side of the mirrorcomponent. For large mirror arrays, e.g. >12×12, such a solution is muchmore cost effective than conventional wire bonding according to theprior art. Flip-chip mounting is not possible without the viatechnology.

In FIG. 1 c there is shown a further prior art device wherein a movableelement such as a mirror 1 is attached to a substrate at 3′ and 3″ bymeans of hinges 4,5 similar to the devices of FIGS. 1 a-b. Actuation isachieved by vias 10′, 11′ of wafer native material extending through thesubstrate wafer SW. The exposed surfaces E1, E2 of the vias 10′, 11′form the electrodes.

The mirror is one representative example of a movable element, i.e. anelement that can be caused to be at least partially displaced ordeflected from a nominal rest position. Other examples (to be describelater) are oscillators, vibrating membranes, optical switches etc.

Electrical Connection into Desired Layers of a Structure ComprisingAlternating Insulating and Conductive Layers

The invention relates to MEMS devices in which it is desired to provideelectrical potentials including ground potential to actuation electrodesat desired locations in a layered structure.

Referring to FIG. 3 a there is shown schematically a layered structurecomprising three (first, second and third, respectively) layers 30, 31and 32 of e.g. silicon or other semi-conducting or conducting material,and interposed between these layers a first insulating layer 33 and asecond insulating layer 34. This layered structure is suitably made fromtwo SOI wafers that have been bonded together, whereby the firstconductive layer 30 constitutes the handle layer and the secondconductive layer 31 constitutes the device layer of a first SOI wafer.

The third conductive layer constitutes the device layer of a second SOIwafer. Thus, as can be understood, the structure shown in FIG. 3 a hasbeen achieved by bonding the two SOI wafers and removing the handlelayer of the second SOI wafer.

Alternatively, instead of using a second SOI wafer, it is equallypossible to bond a second ordinary wafer to provide the third layer. Inthis case the bonded wafer is grinded or polished to the desiredthickness of layer 32. This process is sometimes referred to as a D-SOIprocess.

There is also provided a wafer native via structure 35, 36 extendingthrough the first conducting layer 30. The via structure can comprise awafer native via 35 of heavily doped Si surrounded by an insulatingenclosure 36 so as to provide electrical insulation from the surroundingconductive first layer 30. The wafer native via can be the native wafermaterial suitably doped Methods for making the vias are per se not partof the present invention and will not be discussed herein. Se e.g. theabove mentioned SE-526366.

For MEMS applications in which this kind of layered structures iscommonly used, it is often desired to apply electric potential toselected layers, and sometimes at selected points or areas (i.e.actuation electrodes) in such layers.

According to the invention there is provided a versatile method fortayloring such application of electric potential to the needs at hand.

Thus, the invention provides a method for making electrical connectioninto desired layers of a layered wafer native structure and at the sametime preventing electric coupling into adjacent layers. Using wafernative material has several advantages, as discussed further below.

Referring now to FIG. 3 b, a layered wafer native structure as shown inFIG. 3 a is used, and in a first step a hole 37 is etched through thethird and second conductive layers 32 and 31 and thus also through theinsulating layer 34 as well as through insulating layer 33 and a shortdistance into the via 35. The hole 37 is filled with doped poly-silicon,metal or silicide to provide conductivity. The invention does not limitthe material choice to poly silicon although it is preferred. Any metal,silicide or any conductive material could be used. Poly-silicon ispreferred because it has very similar thermal expansion properties tosilicon. Too large differences in expansion properties could lead tomechanical tension that might “buckle” the mirror. The thermal budgetfor later steps in the process is also affected by most metals. As shownin FIG. 3 b if a potential is applied to the via 35 this potential willbe transferred into both the second 31 and the third 32 layer.

However, in a first embodiment of the invention, illustrated in FIG. 3c, electrical potential is provided through the via 35 and into thethird conductive layer 32 only. In order to achieve this the first SOIwafer has to be processed before it is bonded to the second SOI wafer.Namely there must be provided an insulating enclosure 38 surrounding theportion of the wafer where the poly silicon plug will extend through thesecond layer 31 of the layered structure.

This is achieved by etching a trench 38 in a closed loop into the devicelayer of the first SOI wafer down to the buried oxide layer, andoptionally filling the trench partially or completely with oxide. On theother hand the trenches could be left as they are i.e. filled with air,if they are wide enough so that no electrical breakdown, i.e. electricalcurrent must not be able to pass the trench, can occur. When the two SOIwafers have been bonded together and the handle layer of the second SOIwafer has been removed the procedure discussed with reference to FIG. 3b is performed, i.e. a poly silicon plug 37 is provided through thelayered structure, and the result shown in FIG. 3 c will be obtained. Ifan electric potential is applied to the via in this structure, thepotential will be transferred to the third layer 32 without affectingthe second layer.

In a further embodiment, if it is desired to provide a potentialselectively to the second layer 31, again a trench 38 is made byetching, but in this case it will be made after the SOI wafers have beenbonded together and the handle layer removed from the second SOI wafer.Thus, the trench 38 is made in the third layer 32, and again, like inthe embodiment of FIG. 3 c, at least partially filled with insulatingmaterial. Also in this case it may be possible to leave the trenchesunfilled. Then a hole 37 is etched through the layered structure, asdisclosed with reference to FIG. 3 b, and the resulting structure isshown in FIG. 3 d. Here, an applied potential will be transferred intothe second layer 31 only, and leave the third layer unaffected.

In the shown embodiments the applied potential has been shown to betransferred into entire layers. However, the principle can also be usedfor routing signals or electric potential locally within layers. If forexample the applied potential is to be used for actuating purposes atone specific location within a layer, there could be provided insulatingtrenches forming routing “channels” within the layer in question, suchthat the via can be located at any desired point on the wafer andsignals routed to another point. This is exemplified in FIG. 3 e,wherein one such routing “channel” is schematically shown at 39, with aninsulating enclosure indicated at 38.

Of course the principle of the method is equally applicable if there isonly two layers in the structure, but also for four or even more layers.

Actuation of Deflectable Structures

In devices comprising deflectable structures, such as micro mirrors inprojectors, fiber optical switches, optical amplifiers, loud speakermembranes etc., one of the desired features is to be able to controldeflection and/or plane parallel movement of the structures. Belowreference will be made to mirrors although the principles are applicableto any deflectable structure, such as fiber optical switches, opticalamplifiers and loud speaker elements etc.

There are a number of different ways available to provide the desiredcontrolled deflection. In the first place there has to be some kind of“hinge” or spring structure to which the mirrors are connected. One suchstructure is illustrated above in relation to FIG. 1, thus the mirror isattached to a support structure via a leg or arm that has asubstantially smaller dimension in cross section so as to provide e.g. atorsional deflection. Other types of hinges such as bending or resilientelements (springs) can be used.

In order to extend the movements to movement in two dimensions (aroundtwo perpendicular axes) another type of hinge structure that can be usedis a so called gimbal structure, see FIG. 4 a. A gimbal is a pivotedsupport that allows the rotation of an object about a single axis. A setof two gimbals, one mounted on the other with pivot axes orthogonal, maybe used to allow an object mounted on the innermost gimbal to remainvertical regardless of the motion of its support. In the present contexta gimbal type structure is use to enable deflecting a mirror inbasically all X-Y directions (i.e. 2D actuation) by suitableelectrostatic actuation.

The electrostatic actuation can be achieved in a couple of ways.

The first to be mentioned is by using what can be referred to as “platecapacitor actuation”, similar to FIG. 1 b. Thus, for a mirror that ishinged to e.g. a torsional arm, there is provided one or more electrodesbeneath the mirror at points such that when a potential is applied tothe electrode there will be an electric field between the mirror and theelectrode, causing an attraction towards the electrode whereby themirror will deflect from its rest position towards the electrode. Themirrors can themselves act as electrodes or there can be providedelectrode elements on the gimbals.

The actuation potential is applied to the electrodes by the provision ofvia structures extending through the substrate from the back sidethereof. Thereby there will be no need for providing routing structuresin the same plane as the electrodes, which has the disadvantage ofoccupying space, and may also be fairly complicated from a manufacturingpoint of view.

A disadvantage with the use of plate capacitor actuation is that itrequires high voltages (hundreds of volts), and there is a risk forelectrical break-through if the mirrors (or other elements) are large,thus requiring large deflection, i.e. requires large actuation gap.

The actuation can alternatively be provided by “comb electrodestructures”, which overcome the drawbacks of plate capacitor actuation.Examples of different designs of such comb electrodes are shown in FIGS.4 a-b, as applied to a deflectable micro mirror.

In FIG. 4 a there is schematically illustrated a gimbal hinge structurecomprising plate capacitor actuation.

Thus a mirror 50 is carried by torsion members 52 in a frame 54, whichin turn is carried by torsion members 56 attached to a surroundingsupport structure 58, like in FIG. 4 a.

However, instead of the comb electrodes 60 c, 60 d in FIG. 4 a, beneaththe mirror 50 there are shown in shadow lines two pairs of electrodes 59a and 59 b, respectively. These electrodes are provided in or on thesurface of the substrate 58 in the bottom of the cavity in which themirror is suspended, by vias extending through the wafer and exposing anend surface, such that the end surface constitutes the electrode.Electrodes can alternatively be provided as electrode pads applied onthe end surface of the vias.

When electrodes 59 a are energized they will cause a deflection of themirror in a direction inwards (at the left portion as seen in thefigure) with respect to the plane of the drawing, i.e. a tilting aboutthe Y-axis. Correspondingly energizing of electrodes 59 b will cause adeflection inwards at the right portion, i.e. again a tilting about theY-axis but in the opposite direction. Obviously the opposite part willbe deflected outwards.

Deflection in the other perpendicular directions is provided byenergizing one electrode from each pair, i.e. 59 a and 59 b,respectively. Thus, when this combination of actuation electrodes areenergized the gimbal frame 54 will deflect around its torsion hinges 56,and cause the mirror to deflect correspondingly.

With in-plane comb drive actuators, large action areas over small gapscould be obtained while still offering the possibility for largemovements/deflections. Large areas and small gaps also means low actionvoltage.

Thus, in FIG. 4 b there is schematically illustrated a gimbal hingestructure comprising dual comb electrode actuation.

Thus a mirror 50 is carried by torsion members 52 in a gimbal frame 54,which in turn is carried by torsion members 56 attached to a surroundingsupport structure 58. As an alternative spring members can be used forcarrying the mirror.

As can be seen in FIG. 4 b there are also provided mating combstructures 60 a, 60 b on the frame 54 and on the support 58,respectively, for tilting the mirror about the X-axis, and combstructures 60 c, 60 d provided on the mirror 50 and on the frame 54,respectively, for tilting about the Y-axis. In this way the mirror canbe actuated to move freely in two dimensions, only restricted by thespace available in the cavity in which it is suspended. The combelectrodes on the support (actuation electrodes) are connected to viastructures beneath the structure and extending through the support fromthe back side, like in the example discussed above. The combs on thegimbal frame 54 are made from two of the layers in the structure with aninsulating layer between, whereas the combs on the mirror are made fromone layer, namely the same as the mirror i.e. the lower layer of thetwo. The gimbal combs are energized at an actuation voltage V_(auct) inthe upper portion and connected to ground potential GND at the lower,and the mirror combs are grounded. The combs are connected to viastructures in a suitable layer in the wafer by routing within thetorsion bars 52, 56 and in the frame 54, in accordance with theinvention. This can be seen in FIG. 7 where in the shown embodiment thelower part 72 of the comb structure 70 is coupled to a via 74 throughthe second layer 76 from the top in FIG. 7 and the “plug” 78. Thus, asshown, these combs are provided at different levels in the structure,i.e. they are made in different device layers of the SOI wafers used forthe manufacture.

Thus, when a potential is applied to the actuation electrodes, the combstructure on the mirror will be pulled downwards, but in view of the“fingers” of the combs mating in an interleaved fashion, the deflectioncan be provided in a more versatile manner than in the case whereelectrodes are provided beneath the mirror. For example it will bepossible to make more compact structures using the comb electrodes.

In a further embodiment the hinges are “hidden” beneath the mirrors,which has the advantage that the mirrors can be very densely spaced,i.e. a very compact design can be obtained. This is required for e.g.some optical applications such as adaptive optics whereas much aspossible of the device should be covered by movable mirrors, i.e. thereshould be a minimum area of “dead” surface without mirrors.

Also, for certain wave lengths of light the mirrors often need to becoated with a suitable material. Such reflective coating is mostlyrequired to be present only on the mirror surface itself, and not on thehinges and/or gimbal structures. With the hidden hinge concept theentire wafer can be coated. If hinges are not hidden, there has to beselective coating of the reflective material, e.g. by using “lift-off”,shadow mask stencile shadow mask and other techniques which is much morecomplicated, and does not provide as good yield. Hidden hinges are shownin FIGS. 5 and 6 where a gimbal structure 51 is provided for enablingmovement about two axes. Here the mirror 50 is provided on a post 54.Actuation electrodes 55 are provided by routing in the second layer fromvias 57. It is of course also possible to provide hidden hinges formovement about a single axis.

To make such hidden hinges the process sequence will be different fromthe above described. Reference is made to FIG. 5.

The same basic process involving two SOI wafers can be used, but thehinges are made in the device layer DL1 of the first SOI wafer, and themirror and a post carrying the mirror is made in the device layer DL2 ofthe second SOI wafer. When the SOI wafers have been bonded togetherafter the required structures have been made in the respective wafer, abackside defined opening (DEPRESSION in FIG. 5) is made from the backside in the handle layer (HANDLE in FIG. 5) of the first SOI wafer toprovide a free space in which the hinges can move during deflection.

Alternatively a further SOI wafer is bonded to the structure, this isshown in FIG. 6. Thereby the device layer (DL1 in FIG. 6) thereof isused to provide a spacer member to enable the mirror to be moved(deflected) as desired. The device layer DL0 is in this case etched toprovide a depression which when the wafer is bonded provides the spacefor motion, as indicated in FIG. 6.

The above described processes are applicable also to the provision oflow voltage comb electrode actuators, arranged in dual-axis mirrordesigns containing gimbal structures, although they are not hiddenstructures.

For comb electrodes also a further method is available, see FIG. 7. Inorder to achieve this one has to perform an underetch under the hingestructures after the wafers have been bonded together. Thereby the hingestructure is protected by an oxide layer and a silicon etch is appliedwhereby material is removed also from under the hinge so as to provide afree space (Alt-1 RECESS) for deflection. The handle of this furtherwafer is removed by etching when the other wafer(s) have been bonded.

It is also possible to provide a larger recess (Alt-2 RECESS) under themirror by removing additional material from the handle layer below thedevice layer DL0.

Also, it is possible to provide the recess (Alt-3 RECESS) by a DeepReactive Ion Etch (DRIE) through the DL0 layer.

In a particular application, schematically shown in FIG. 8 a movableelement such as a mirror 80 is provided above a cavity 82 in a substrate84. In the shown embodiment the mirror 80 is suspended at its corners 86to the substrate by means of spring members 88. The spring members 88are only schematically represented in FIG. 8 at 88. However, in onembodiment they can be “meander” shaped as shown in the magnified insertin FIG. 8. However, any shape that would provide some spring action orresilience would be usable.

Four electrodes 89 are provided beneath the movable element 88, but onesingle electrode may be sufficient. Now, if the electrodes are energizedthe movable element will be electrostatically attracted by theelectrodes and thereby translated inwards in the cavity, i.e. a planeparallel displacement will occur. This application can be used as anoptical amplifier, phase changer or other adaptive switch.

It is not strictly necessary to provide four electrodes, any number fromone and more could be used, as long as the attractive force can beapplied evenly such that the actual plane parallel translationdisplacement is ascertained. For example one large electrode having asurface area corresponding to at least a part, preferably a major part,of the surface of the movable element would function. The electrodes caneither be formed by the exposed surface of the via in the bottom of thecavity in which the mirror is suspended, or they could be provided asmetal pads on the vias.

In the description given above the electrodes have only been used foractuation purposes. However, in a further embodiment shown in FIG. 9, apiezo electric film 90 (made from e.g. PZT, ALN, piezo electric polymersand other materials known to the skilled man) is deposited on aschematically shown movable element 92 (e.g. a membrane) which issuspended above a cavity 94. The piezo film has one electrode 91, 93attached to each side. The electrodes are connected to vias 95, 96,respectively, using a layered structure in accordance with the presentinvention.

The reverse effect of the piezo material can be used for picking upsignals/energy from the movable structure. In this mode it can be usedfor sensors or for energy harvesting applications. Similar to a combdrive structure, which can also be used in a sensing mode, at least twoseparate routing layers according to the present invention are needed.

Now methods of making the structures disclosed above will be described.The description will be given with reference to mirrors and mirrorarrays. However, the principles used are equally applicable for othermovable and deflectable elements such as those mentioned earlier, namelyare oscillators, vibrating membranes for loud-speakers, optical switchesetc.

Method of Making Micro-Mirrors

In a process for manufacturing deflectable micro mirrors and/or arraysof such mirrors, at one stage in such a process and before the actualmirror structures are manufactured, two wafers (a first wafer and asecond wafer) are bonded together in a controlled atmosphere, e.g.vacuum. One of the wafers (first wafer) thereby has a depression formedin it to provide the necessary space in the final structure for thedeflectable mirrors to move freely during deflection. The second wafer(suitably a SOI wafer) provides a “lid” over the depression.

Thus, after bonding together of the wafers the depression in the firstwafer will be sealed off by the second wafer and thus a controlledatmosphere (e.g. vacuum) cavity is formed. In subsequent steps of theprocess, machining of the second wafer is performed for making the finalmirror structures. The mirror structures comprise an actual mirror partwhich is relatively thick and rigid, and a hinge part.

However, the mirror can have different thicknesses, for providingdifferent resonance frequencies—thick mirrors mean large mass and lowfrequency; thin mirrors mean small mass and high frequency. Thefrequency requirements may be contrary to the flatness requirements.Thinner mirrors can more easily be bent due to mechanical impact. It ispossible to make a mirror with a rigid frame part and remaining areasthinned down to provide low mass and higher rigidity. Also the hingescan be thinned down to varying degrees.

The hinge will be substantially thinner than the mirror in someembodiments, in order to provide the required flexibility for the hingeto function as desired. In particular the hinge can be provided as socalled gimbal structures.

However in other cases, e.g. when a torsional effect is desired, thehinge can have the same thickness as the mirror, but will then have alateral extension (i.e. in the transverse direction of the hinge) thatis relatively small.

Manufacturing of these structures are made by suitable masking andetching of the second wafer. However, the process steps of making themirror structures are performed in an atmosphere having a differentpressure (normally higher) than the pressure prevailing inside thecavity. Thus, since there will be a pressure difference across the“lid”, when the etching process “breaks through” the SOI wafer toprovide the free-hanging hinged mirrors, there will be a sudden pressurelevelling. This pressure levelling yields strong forces such that thedelicate hinge structures mirrors for the mirrors very easily breaks andthe mirrors fall out from the device, with extremely low yields as aresult.

According to the present invention there is provided for a controlledventing of the structure such that the pressure levelling will be verysmooth and no strong forces will be exerted on the delicate hinges.

The solution according to one embodiment of the invention isschematically illustrated in FIGS. 2 a-d.

FIG. 2 a shows a first wafer 20 (substrate wafer) having a depression 21formed therein, bonded to a SOI wafer 22 comprising a device layer 23,an oxide layer 24 and a handle layer 25. In the device layer 23 of thesecond wafer suitably a thinned down portion 23′ has been made to definethe thickness of a hinge that is to connect the mirror with thesupporting structures in the finished product.

The handle layer 25 is removed (FIG. 2 b) and after suitable masking MVa first etching is performed to provide a vent hole precursor structure26 in the remaining device layer 23. This precursor structure isessentially a hole or a groove, which has a predefined depth, i.e. whichextends down into the device layer.

Then there is provided suitable masking MMH (FIG. 2 c) to define themirror 27 and its hinge 28 structures by opening up a contour (at 29) inthe mask, and a second etch is performed. This is schematicallyillustrated in FIG. 2 c. Thereby the vent hole precursor structure 26will open up the cavity 21 with controlled atmosphere (e.g. vacuum)before the etch has removed so much material in the contour trench 29from the device layer that the material in the trench at 29 has becomeso thin that it might break due to the forces exerted when pressure islevelled out through the vent hole.

The etch is continued until the mirror 27 is free-etched, i.e. thedevice layer is etched through in the contour trench at 29.

In an alternative embodiment the entire process can be performed in onestep. This is possible by dimensioning the vent hole precursor structure26 to be large enough that the etch will excavate material therein at afaster rate than in the trench defining the mirror which in its turnwill be etched faster that the hinges, like in the previous embodiment.This is schematically shown in FIG. 2 d, which shows a larger vent hole26 than in FIG. 2 c.

In particular, the invention provides a method of making a layered MEMSstructure comprising alternating conductive and insulating layers and anelectrical feed-through structure for routing electrical signals orpotentials to selected points or areas within a selected layer in saidstructure, the method comprising the steps of: providing a first SOIwafer having a via structure provided in the handle layer and extendingtherethrough to the insulating buried oxide layer; providing a secondwafer; bonding the wafers together; thinning the second wafer; creatingan insulating structure in the form of a trench running in a closedloop, in the device layer of either of the two wafers, the materialinside the closed loop at least partially overlapping the via structurewhen the wafers are bonded together; making a hole through the devicelayers of the bonded wafers, said hole extending down into the viastructure and filling said hole with conductive material, suitably polysilicon, to provide electrical connection.

Also there is provided a method of making a device having a movablemicro element provided above a cavity in said device, comprising atleast one movable element and at least one hinge for said element andactuation electrodes for causing movement of said micro element saidelectrodes being connected to routing structures made by the method asclaimed in claim 20 or 21, the method comprising the steps of: bondingtogether a first wafer, having a depression formed therein for providingenough space for a micro element to be able to deflect from a restposition as desired without contacting surrounding structures, and asecond wafer from which the micro element subsequently is to be made,whereby a closed cavity is formed between the wafers; providing maskstructures to define i) the micro element, and ii) the hinge structure,etching through said mask structures in a controlled way such that thevent hole opens up before the micro element and hinges have beenfinished.

Furthermore, there is provided a method of making a MEMS structurecomprising alternating conductive and insulating layers and anelectrical feed-through structure for routing electrical signals orpotentials to selected points or areas within a selected layer in saidstructure, the method comprising the steps of: providing a first SOIwafer having a via structure provided in the handle layer and extendingthere through to the insulating buried oxide layer; providing a secondwafer; bonding the wafers together; thinning the second wafer; creatingan insulating structure in the form of a trench running in a closed loopin the device layer of either of the two wafers, the material inside theclosed loop at least partially overlapping the via structure when thewafers are bonded together; making a hole through the device layers ofthe bonded wafers, said hole extending down into the via structure andfilling said hole with conductive material, suitably poly silicon, toprovide electrical connection.

Finally there is also provided a layered MEMS structure comprisingalternating conductive and insulating layers, and further comprising: avia structure in a first outer layer; a conductive plug, preferably ofpoly silicon extending through the other layers and into the viastructure to provide conductivity through the layers; and an insulatingenclosure surrounding said conductive plug in at least one selectedlayer of said other layers so as to insulate the plug from the bulk saidselected layer.

We claim:
 1. A device comprising a deflectable structure (1, 2, 3, 4,5), provided on a substrate (SW), said substrate comprising a pluralityof electrodes (59 a, 59 b; 6, 7), beneath the deflectable structure atpoints such that when a potential is applied to the plurality ofelectrodes an electric field is generated between the deflectablestructure and the plurality of electrodes, causing an attraction towardsthe plurality of electrodes whereby the deflectable structure isdeflected, wherein the plurality of electrodes (59 a, 59 b; 6, 7)beneath the deflectable structure comprises via structures extendingthrough the substrate (SW) from its back side, the via structures havingsurfaces facing the deflectable structure forming the plurality ofelectrodes,wherein the deflectable structure is a mirror suspended by agimbal structure to a surrounding support structure on the substrate,the gimbal structure having gimbals, each of the gimbals allowingrotation of the mirror about an axis, the device further comprisinginterleaved comb shaped electrodes on selected parts of the gimbalstructure and on the support structure respectively, such that when theplurality of electrodes on the support structure are energized adeflection of the mirror(s) takes place.
 2. The device of claim 1,wherein the deflectable structure itself is provided so as to functionas an electrode.
 3. The device of claim 1, wherein the plurality ofelectrodes are provided such that an electric potential can be appliedto them, whereby an electric field is generated between the deflectablestructure and the electrode(s).
 4. The device of claim 1, wherein thedeflectable structure is suspended so as to have one part attached tothe substrate and one part freely movable within a cavity.